This invention relates to blind fasteners for securing panels and sheet material together, and more particularly, to an improved drive nut design for retaining the drive nut to a screw.
Blind fasteners are commonly used to secure two sheets together when it is otherwise impossible to access the underside (blind side) surface of one of the sheets. Such fasteners have wide application in aircraft and space vehicle assembly. Due to the vibrations and sonic fatigue encountered in these environments, it is necessary to create a fastener of enduring strength and reliability.
The previously available blind fastener that this invention improves upon comprises: (1) a screw, (2) a sleeve, (3) a nut, and (4) a drive nut. Examples of the previously available blind fasteners are disclosed in U.S. Pat. Nos. 4,772,167 and 4,747,202. Generally, the screw has an externally threaded surface that allows the sleeve, the nut, and the drive nut, which are each internally threaded, to be placed onto the screw. The blind fastener is inserted into aligned apertures of the sheets that are being secured together, and after installation is complete the sleeve and the nut will clamp the sheets together.
The screw has the shape of a long threaded bolt with an enlarged head at one end of the screw, and at the end opposite of the head is two machined flats. The machined flats are wrenching surfaces that allow a tool to rotate the screw during assembly and installation of the blind fastener. The screw also has a frangible groove at a predescribed position on the threaded section that is machined to a diameter smaller than the minor diameter of the external thread. The purpose of this frangible groove is to prevent over torquing and/or excessive upsetting of the sleeve during installation by serving as a breakneck. When a certain installation load is achieved, the frangible groove prevents overloading by failing first in torsional shear and then breaking away from the assembly.
The sleeve has a cylindrical shape and is made of a malleable material. The purpose of the sleeve is to expand radially and abut against the blind side surface of the sheet during installation. The nut has a cylindrical shape and resembles a flush head or protruding head bolt, with the exceptions that it is internally threaded throughout, and on the top of the head has a driving recess. Further, the nut is dimpled around its diameter in such a manner as to provide friction on the screw to prevent unloosening of the screw once the fastener is installed.
The driving nut resembles a traditional hexagon nut and has the same internal threading as the nut. On one end of the drive nut is a chamfered angle that begins from the external hexagon shape and ends at a predetermined counterbore that is larger than the internal thread diameter. Further, the drive nut has an annular ridge that is made of a malleable material that allows deformation of the annular ridge into the head and the recess of the nut.
The assembly process of the previously available blind fastener consists of the sleeve being placed over the screw until it abuts the head of the screw, followed by the nut being threaded onto the screw until it is seated against the sleeve. The nut is then dimpled on the outer wall by a physical deformation process that places some of the nut material against the internal screw to prevent loosening. The drive nut is then threaded onto the screw until it is seated against the head of the nut. The installation process of the blind fastener is accomplished by use of a tool adapted to hold the drive nut stationary and simultaneously fit over the machined flats on the screw. The tool prevents the drive nut from rotating, while at the same time rotates the screw. As the screw rotates, the sleeve is pulled towards the blind side of the sheet material. The malleable sleeve then abuts the sheet material and begins to deform into an expanded diameter. Meanwhile, at the opposite end of the blind fastener assembly, the annular ridge of the drive nut begins to deform and flatten against the nut head. As the torsional and compressive load increases, the annular ridge of the deformable drive nut begins to rotate as much as 180xc2x0 degrees before it penetrates the recess of the head of the nut which is evidenced by head marking and material smearing.
The drive nut behaves as a xe2x80x9cjam nutxe2x80x9d by forcibly deforming against the nut head to prevent the nut from rotating. At a certain torsional and compressive load the screw stops rotating and the breakneck frangible groove fails, causing the drive nut and remaining screw to fall away.
The problems with the presently available blind fastener derive from the deformable drive nut. Optimum installation performance and reliability are not achieved from the blind fastener because the deformable drive nut allows undesired rotation and xe2x80x9cjam nut effectxe2x80x9d. The deformable drive nut rotates upon the head of the nut as the annular ridge of the drive nut deforms into the recess of the nut head. This deformation process causes rotation of the nut and smearing or scraping of the nut head. This result is not only visually apparent, but can also deteriorate the nut""s corrosion resisting properties and damage the plating under the head and grip area of the nut. Finally, the xe2x80x9cjam nut effectxe2x80x9d of the deformable drive nut causes large variations in the required installation loads. This can result in premature screw break off and inconsistencies in the amount of sleeve material that deforms into an expanded diameter, thus compromising the integrity of the blind fastener.
Consequently, a need exists for a blind fastener with a non-deformable drive nut that has positive engagement into the head of the nut thereby eliminating rotation of the nut and all of the problems associated with undesired rotation and xe2x80x9cjam nut effectxe2x80x9d.
The present invention is directed to a blind fastener with an improved drive nut and assembly process. The blind fastener of the present invention includes a screw, a sleeve, a nut, and a drive nut. The screw, the sleeve, and the nut of this blind fastener are similar to those of the previously available blind fastener discussed above. The improvements of the present invention relate to a new structure for the drive nut and a new assembly process for the blind fastener. The new drive nut and assembly process significantly improve the reliability of the blind fastener by eliminating the inconsistencies associated with forcibly deforming the drive nut into the head of the nut.
The new drive nut of this invention has protruding nibs that mate precisely with a recess located in the nut head. The protruding nibs of the new drive nut are not deformable and serve to replace the deformable annular ridge that was used in previous drive nut assemblies. The configuration of the protruding nibs and the recess in the nut head can be of many forms, so long as both parts are capable of being mated. The protruding nibs are fabricated at one end of the drive nut and are perfectly engaged to resist any installation torsional loading. The drive nut can have an internally threaded bore to matingly engage the screw, or in an alternative configuration can have a smooth bore for placement over the screw. For smooth bore configurations the drive nut is held in place on the screw by a retaining ring, insert or plug positioned on the drive nut opposite the protruding nibs. Alternatively the drive nut can be held onto the screw by an adhesive. Yet another alternative for retaining the drive nut with a smooth bore onto the screw is by staking a small portion of the drive nut into the threads on the screw. Once the new assembly process for the blind fastener has been completed, the drive nut will have been mated with the nut head to prevent any rotation of the nut during installation of the blind fastener into the sheets being secured.
The assembly process of the blind fastener comprising the new drive nut also differs from that previously utilized. The assembly process will now include placing the sleeve over the threads of the screw to a position adjacent the screw head. The nut is then partially threaded onto the screw until the internal threads within the region of the enlarged nut head have not been completely threaded onto the screw. For an internally threaded drive nut, the drive nut""s protruding nibs are next aligned and physically engaged into the mating recess in the nut head. The mated drive nut and nut are next rotated which results in the drive nut and nut being physically locked together and being threaded onto the screw as a single unit. Once the nut is adjacent to the sleeve, the assembly is completed by positioning the screw, the sleeve, the nut, and the drive nut so the nut can be dimpled. The dimpling process places a small physical deformation on an outer surface of the nut which displaces a small portion of the nut material against the internal screw to cause sufficient friction to prevent loosening.
For a smooth bore drive nut, the nut is threaded all the way onto the screw adjacent the sleeve. The drive nut is then slid over the screw until the nibs engage the recess in the head of the nut. The drive nut is held in place by placing the retaining ring, insert or plug on the backside of the nut, or alternatively by applying on adhesive into the smooth bore. Yet alternatively, the drive nut is held onto the screw by staking an end portion of the drive nut into the threads of the screw by a hollow mandrel. The drive nut can alternatively be staked onto the screw on one or more locations along the side of the drive nut.
Once the new assembly process is complete, the blind fastener having the new drive nut is installed into the sheets to be secured by means similar to those used with the previously disclosed blind fastener. Namely, a tool with a means for holding the drive nut stationary, and at the same time rotating the screw, is used to cause the sleeve to deform against the blind side surface of the sheet and secure the sheets between the nut and the sleeve.
As the sleeve deforms against the blind side of a sheet during installation, the drive nut will not deform into the nut head. The mating connection between the drive nut and nut head prevents smearing and scraping of the nut head and preserves its corrosion resistant properties. Unlike the previously available drive nut having a deformable annular ridge, the drive nut of this invention has no xe2x80x9cjam nut effectxe2x80x9d. The protruding nibs on the drive nut eliminates the unpredictability of installation loads placed on the screw by not forcibly deforming against the nut head. This in turn greatly reduces the risk of premature screw break off and increases consistency in the amount of sleeve material that will be deformed against the sheet. These new drive nut designs improve the consistency of loading by at least 30 percent or more. This relates to a significant improvement in reliability of the installation process and reduces variability.
Other objects, features and advantages of the invention will be apparent from the following detailed description taken in connection with the accompanying drawings.